In vivo delivery of proteins in biologically relevant forms and amounts has been an obstacle to drug and vaccine development for decades. One solution that has proven to be a successful alternative to traditional protein delivery approaches is the delivery of exogenous nucleic acid sequences for production of proteins in vivo. Gene transfer vectors ideally enter a wide variety of cell types, have the capacity to accept large nucleic acid sequences, are safe, and can be produced in quantities required for treating patients. Viral vectors are gene transfer vectors with these advantageous properties (see, e.g., Thomas et al., Nature Review Genetics, 4: 346-358 (2003)). Furthermore, while many viral vectors are engineered to infect a broad range of cell types, viral vectors also can be modified to target specific cell types, which can enhance the therapeutic efficacy of the vector (see, e.g., Kay et al., Nature Medicine, 7(1): 33-40 (2001).
Viral vectors that have been used with some success to deliver exogenous proteins to mammalian cells for therapeutic purposes include, for example, Retrovirus (see, e.g., Cavazzana-Calvo et al., Science, 288 (5466): 669-672 (2000)), Lentivirus (see, e.g., Cartier et al., Science, 326: 818-823 (2009)), Adeno-associated virus (AAV) (see, e.g., Mease et al., Journal of Rheumatology, 27(4): 692-703 (2010)), Herpes Simplex Virus (HSV) (see, e.g., Goins et al., Gene Ther., 16(4): 558-569 (2009)), Vaccinia Virus (see, e.g., Mayrhofer et al., J. Virol., 83(10): 5192-5203 (2009)), and Adenovirus (see, e.g., Lasaro and Ertl, Molecular Therapy, 17(8): 1333-1339 (2009)).
Despite their advantageous properties, widespread use of viral gene transfer vectors is hindered by several factors. In this respect, certain cells are not readily amenable to gene delivery by currently available viral vectors. For example, lymphocytes are impaired in the uptake of adenoviruses (Silver et al., Virology, 165: 377-387 (1988), and Horvath et al., J. Virology, 62(1): 341-345 (1988)). In addition, viral vectors that integrate into the host cell's genome (e.g., retroviral vectors) have the potential to cause insertion mutations in oncogenes (see, e.g., Cavazzana-Calvo et al., supra, and Hacein-Bey-Abina et al., N. Engl. J. Med., 348: 255-256 (2003)).
The use of viral vectors for gene transfer also is impeded by the immunogenicity of viral vectors. A majority of the U.S. population has been exposed to wild-type forms of many of the viruses currently under development as gene transfer vectors (e.g., adenovirus). As a result, much of the U.S. population has developed pre-existing immunity to certain virus-based gene transfer vectors. Such vectors are quickly cleared from the bloodstream, thereby reducing the effectiveness of the vector in delivering biologically relevant amounts of a gene product. Moreover, the immunogenicity of certain viral vectors prevents efficient repeat dosing, which can be advantageous for “boosting” the immune system against pathogens when viral vectors are used in vaccine applications, thereby resulting in only a small fraction of a dose of the viral vector delivering its payload to host cells.
Thus, there remains a need for improved viral vectors that can be used to efficiently deliver genes to mammalian cells in vivo. The invention provides such viral vectors.